1. Field of the Invention
The present invention relates to an apparatus and method for processing exhaust gas from an internal combustion engine and, more particularly, to an apparatus for processing exhaust gas from an internal combustion engine including a storing catalyst, in which fuel is burned in an exhaust gas stream in order to produce a reducing atmosphere in the exhaust gas.
2. Description of Related Art
Legal regulations, which control emissions from motor vehicles for protection of health and the environment, are continually being enhanced according to the advancing state of the art. Besides, e.g., the use of appropriately modified fuel in order to fulfill these regulations, NOx-storing catalysts, which store nitrogen oxides during lean motor vehicle operation, i.e. when air is present in excess (xcex greater than 1) in the exhaust gas during the combustion process, are employed. Under these conditions only minimal carbon monoxide and fuel emissions occur, however comparatively high NOx emissions occur. Particularly the exhaust gas contains nitrogen monoxide (NO).
The NOx-storing catalysts are able to store NOx-compounds because of their coating, which absorbs NOx from the exhaust gas, until the NOx-storing catalysts are completely filled so that they cannot store any more NOx. For regeneration of the NOx-storing catalysts a reducing exhaust gas atmosphere (xcex less than 1) is produced, which is, for example, not typical for modern diesel engines.
An incomplete combustion of fuel, in which carbon monoxide and fuel material (HC) are emitted, results from an under-stoichiometric air/fuel ratio (xcex greater than 1). These reducing agents and hydrogen (H2) regenerate the NOx-storing catalysts during a short regeneration phase, in which the stored NOx-compounds together with the carbon monoxide (CO) and fuel materials (HC) and/or the hydrogen (H2) in the catalyzer are broken down into carbon dioxide (CO2) and water (H2O), above all to nitrogen (N2).
Generally an Otto engine can be operated fairly easily under both lean and rich conditions. Rich operation in a diesel engine is comparatively difficult so that regeneration of the NOx-storing catalysts must be performed by means of additional auxiliary systems.
Currently Diesel fuel is directly fed to an exhaust gas stream and burned for producing a reducing exhaust gas atmosphere. This Diesel fuel is injected and ignited only in a simple manner. The input fuel burns comparatively uncontrollably so that undesirable carbon formation occurs and the combustion is only influenced with some difficulty.
It is an object of the present invention to provide an apparatus for treatment of exhaust gas from an internal combustion engine including a storing catalyst and an additional fuel combustor, in which substantially no carbon is formed, even with a strongly under stoichiometric air-fuel ratio.
It is another object of the present invention to provide a method of processing exhaust gas from an internal combustion engine to reduce pollutant emissions from the engine, even under strongly fuel rich operating conditions.
These objects and others, which will be made more apparent hereinafter, are attained in an apparatus for processing exhaust gas from an internal combustion engine to reduce or eliminate pollutant emissions, which apparatus includes a storing catalyst and means for burning fuel in an exhaust gas stream during lean operation in order to produce a reducing atmosphere in the exhaust gas.
According to the invention the apparatus for processing the exhaust gas includes aerodynamic means for controlling the burning of the fuel in the exhaust gas stream in order to produce the reducing gas atmosphere.
The method of processing exhaust gas to better reduce and eliminate pollutant emissioris includes burning fuel in the exhaust gas stream during lean operation and controlling the burning of the fuel in the exhaust gas stream by means of an aerodynamic device.
Additional aspects and embodiments of the invention are claimed and described in the appended dependent claims.
The apparatus according to the invention is characterized by an aerodynamic controller for the combustion of fuel in the exhaust gas stream.
This sort of controller guarantees that no undesired carbon formation takes place even under under-stoichiometric air-fuel conditions. At the same time good control of the burning or combustion of fuel in the exhaust gas stream is provided, for example in periodic operation of the apparatus.
Control of the burning rate preferably occurs, so that the air-fuel ratio can be flexibly adjusted to differing conditions.
The combustion preferably occurs with fuel, which is also used in the internal combustion engine, i.e. generally with Diesel fuel, or fuel derived from it, for example with reformed hydrogen derived from the fuel. However other fuels can be used, which are burned according to the invention and thus, for example, produce a carbon monoxide and/or hydrogen rich treatment gas for the regeneration. A substantially incomplete oxidation of the fuel occurs in the combustion occurring in the exhaust gas stream according to the invention.
In a special embodiment of the invention the aerodynamic controller includes at least one flow-restricting device, so that the exhaust gas stream is accelerated. The turbulence intensity and the speed distribution are controlled so that advantageous spinning or swirling and mixing of the burning fuel-air mixture and the exhaust gas occur. An orifice, a screen or a baffle plate can be provided as the flow-restricting device and/or flame holder.
The aerodynamic means for controlling the combustion includes at least one swirling device, so that an additional spatial localization and stabilization of the flame occurs. For example, swirling or spinning devices can be provided both upstream and downstream of the fuel inlet. Because of these features a carbon-poor combustion is guaranteed at under-stoichiometric conditions. The combustion conditions are directly influenced in a positive manner in combination with the above-described flow-restricting device.
Advantageously the aerodynamic controller includes at least one recirculating device so that a part of the exhaust gas from the reaction or combustion chamber is fed back to the burning flame, which reduces the average temperature of the flame and local temperature spikes, whereby the NOx fraction of the exhaust gas decreases. Furthermore the under-stoichiometric combustion is additionally spatially stabilized because of the recirculating device, especially in cooperation with the above-described flow-restricting and swirling devices. Especially because of the recirculation exhaust gas that has scarcely any carbon results from combustion in near- or under-stoichiometric air/fuel mixtures.
In other embodiments the apparatus has at least one bypass, which receives a portion of the exhaust gas stream from the internal combustion engine, so that a definite exhaust gas volume flow always passes through the apparatus according to the invention at different operating conditions of the internal combustion engine. Because of this feature nearly constant flow conditions are guaranteed in the apparatus so that the combustion is additionally controllable in an advantageous manner.
The exhaust gas volume flow is adjusted reliably to other combustion conditions by means of an appropriate exhaust gas flow control means so that more or less exhaust gas is conducted through the apparatus as required, which is advantageous, for example, during combustion ignition.
Preferably the apparatus includes an air supply device so that the regeneration of the NOx-storing catalyst is, for example, performed during stoppage of the internal combustion engine. At the same time this embodiment facilitates a definite adjustment of the oxygen content in the exhaust gas stream to a predetermined value.
The apparatus preferably includes means for cooling so that an overheating of the apparatus, especially the fuel supply, is prevented.
Preferably an oxidation catalyzer is arranged between the apparatus and the storing catalyzer, in which nitrogen monoxide (NO) is converted to nitrogen dioxide (NO2), so that the NOx-storing catalyzer is supplied nitrogen dioxide almost exclusively. Because of that feature the storing of the exhaust gas nitrogen in the NOx-storing catalyzer is optimized, since the nitrogen dioxide (NO2) is converted to the nitrate (NO3) and that is absorbed.
In a special embodiment of the invention a particle filter is arranged between the storing catalyzer and the apparatus for burning fuel in the exhaust gas stream. This particle filter decreases particle emissions from both the internal combustion engine and the apparatus for burning fuel in the exhaust gas stream so that they do not reach the storing catalyzer.
Advantageously the particle filter is regenerated by the apparatus according to the invention, whereby the regeneration of the particle filter occurs at over-stoichiometric or lean conditions (xcex greater than 1). Thus the apparatus is operated in an advantageous manner at the two different combustion air/fuel ratios. A reducing process gas is produced for storing catalyst regeneration (under-stoichiometric operating conditions) and the particle filter is cleaned by burning off particles in large spacing (over-stoichiometric operating conditions).